Artificial turf having a longer turf length than ordinary one, so-called long pile artificial turf, has spread through various athletic sports facilities, such as a soccer field, a rugby field, and a baseball field, as an artificial turf surface having the characteristics close to natural turf by filling the space between the piles with a filler.
As a filler for this type of artificial turf, for example, an elastic granular material of a rubber chip (a crushed article of a waste tire or industrial rubber such as EPDM) or a thermoplastic elastomer (such as a PE-based elastic resin) is preferably used.
Incidentally, since a rubber chip prepared using a waste tire itself is colored in black with carbon, it easily absorbs sunlight, and the surface temperature of the artificial turf may become 60° C. or more under blazing heat in summer or the like. As a result, exercise on the artificial turf in summer may impose a large load to a player, impairing comfort. Further, in some cases, the artificial turf cannot be used over a long period of time since the surface temperature is kept high by a filler which has absorbed heat even after sunset.
Therefore, for example, in the invention described in Patent Literature 1, titanium oxide having light-reflecting properties is added to a filler to thereby reflect light to suppress an increase in temperature of the surface of artificial turf, by which a certain temperature increase-suppressing effect is obtained compared with the case where a black rubber chip is used; however, it is hard to say that the increase in temperature can be sufficiently suppressed.
Therefore, as a method of further enhancing a cooling effect, for example, Patent Literatures 2 and 3 disclose a method of suppressing an increase in temperature by evaporation heat, in which a part of a primary backing of artificial turf and a filler is allowed to contain a water-absorbing resin, which is sprinkled with water to store water therein.
However, this method requires time for maintenance because the surface of the artificial turf has to be periodically sprinkled with water, and since a problem may occur in the properties of artificial turf itself without an accurate control of the amount of the water-absorbing resin and the amount of water sprinkled, management of the state will take further labor.
As a method of solving these problems, for example, Patent Literature 4 discloses a technique in which the inner part of turf yarn (pile) is formed into a hollow straw shape; the pile is implanted in a primary backing in a U-shape so that both open ends may farm the tips of the pile (refer to FIG. 4 of Patent Literature 4); and further the pile on the primary backing side is broken so that the water stored on the bottom of the artificial turf can be emitted from the tip side of the pile to the surface of the artificial turf by the capillary action.
According to the technique, the artificial turf is trampled to allow the water to leak out from the turf tip, thereby capable of suppressing an increase in temperature over a long period of time. However, the method described in Patent Literature 4 has been designed as a surface for artificial turf skiing areas in summer and intends to always keep the surface of the artificial turf in a wet state, in order to protect a fallen person from an abraded wound and to improve the sliding of the ski.
Therefore, when the artificial turf of Patent Literature 4 is applied to a surface for general athletic fields such as a soccer field and a baseball field, it will be slippery and may, conversely, largely impair the playability since the surface of the artificial turf is wet. Further, since all of the turf yarns are constituted by straw-shaped piles in this artificial turf, it has been difficult to improve the characteristics as a surface for sports in spite of good temperature suppression effect.